Retroviruses have often been used for gene therapy because of their capacity for the long-term expression of transgenes via stable integration into the host genome. However, retroviral integration can also result in the transformation of normal cells into cancer cells, as demonstrated by the incidence of leukemia in a recent retroviral gene therapy trial in Europe. This unfortunate outcome has led to the rapid initiation of studies examining various biological and pathological aspects of retroviral integration. This review summarizes recent findings from these studies, including the global integration patterns of various types of retroviruses, viral and cellular determinants of integration, implications of integration for gene therapy and retrovirus-mediated infectious diseases, and strategies to shift integration to safe host genomic loci. A more comprehensive and mechanistic understanding of retroviral integration processes will eventually make it possible to generate safer retroviral vector platforms in the near future.